Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure roll) serves to press the receiver sheet into contact with the fuser roll.
The fuser roll usually comprises a rigid core covered with a resilient material, which will be referred to herein as a "base cushion layer." The resilient base cushion layer and the amount of pressure exerted by the pressure roll serve to establish the area of contact of the fuser roll with the toner-bearing surface of the receiver sheet as it passes through the nip of the pair of rolls. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roll. The degree of hardness (often referred to as "storage modulus") and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is condensation-crosslinked poly(dimethylsiloxane) elastomer. "Poly(dimethylsiloxane)" will sometimes be alternatively referred to herein as "PDMS".
Fluorocarbon resins like polytetrafluoroethylene (PTFE) or a copolymer of PTFE and perfluoroalkylvinylether, or fluorinated ethylenpropylene have excellent release characteristics due to very low surface energies, high temperature resistance, and excellent chemical resistance. Fluorocarbon resins are, however, less flexible and elastic than polysiloxane elastomers and are therefore not suitable alone as the surface of the fuser roll.
Polyfluorocarbon elastomers, such as vinylene fluoride-hexafluoropropylene copolymers, are tough, wear resistant and flexible elastomers that have excellent high temperature resistance, but relatively high surface energies, which compromise toner release. This can be seen in the comparative examples below.
Some fusing systems use a release oil, such as a PDMS oil, to prevent offset, that is, to aid the roll in releasing from the toner it contacts during the fusing operation. During use, the oil is continuously coated over the surface of the fuser member in contact with the toner image. Unfortunately, the materials that are used for the fusing member can be affected by the oil, causing swelling. Swelling causes an undesirable change in the nip uniformity and undesirable wear characteristics. The swelling of the fuser roller materials has been a continuing problem.
In U.S. Pat. No. 4,568,275 there is described a fuser roll having a layer of a fluorinated rubber and a fluorinated resin powder. However, the fluorinated rubber that is disclosed is water dispersable and it is known that the mixture phase-separates on coating so that the fluorinated resin that is used comes to the surface of the layer. In a comparative example below, it is shown that this composition has marginal release properties and marginal wear characteristics.
In U.S. Pat. No. 5,253,027, there is described a fluorinated resin in a silicon elastomer. However, composites of this type exhibit unacceptable swell in release oils.
Thus, it has been extremely difficult to provide a fuser roller with, at the same time, good wear resistance, good release properties and low swell when exposed to release oils. It is to a solution to this problem that the present invention is directed.